There's currently a guy trying to make an extruder based printer using glass filled nylon. If, and this is a big if, it can be made to work, then that could be used to make a lower just like the polymer lowers that have been coming into the limelight recently.

The EOSINT M280 can build a firearm, save for the barrel, right now. Plug in the STL files, wait for it to sinter the powder into the proper shape, do a modicum of finishing work on bearing surfaces, and within 24 hours, you have an honest to god firearm on par with current manufacture. That machine can print in everything from aluminium to steel to titanium and a few exotic super-alloys. It also costs 750,000 dollars for a full installation. The material costs are also very high, with each KG of the aluminium sintering powder sitting at just over 50% more than the same weight of billet in the same alloy.

Now, for a few hundred rounds, you may be able to get away with PLA or ABS printers, provided they are accurate enough and use a small enough layer depth. There's a machine that costs 2200 dollars called the Aluminatus that has taken the reprap concept as far as it can go currently. http://trinitylabs.com/pages/aluminatus-overview For the price, it has the best accuracy and layer resolution. With a good, solid model, you could print an AR lower that would perform for a little while. If you're willing to get creative with some very long and skinny screws, you might be able to make an AR that stays together for several thousand rounds until the buffer tower and takedown pin holes begin to warp.

No matter what printer you get, though, remember that you are printing in an inferior material to the original. Even if you are printing a glock frame, you do not have the benefit of millions of dollars worth of polymer research and development. Use your head, over build everything, over engineer everything, and do not be afraid to deviate from the original plans. Modify, improve, break, repair and learn along the way. One day very soon, someone will crack the code to a reliable AR lower. Might as well be you.